Due at least in part to high crude oil prices, environmental concerns, and future fuel availability, many internal combustion engine designers have looked to replace crude oil fossil fuels, e.g., gasoline and diesel, with other fuels for powering internal combustions engines. Desirably, by replacing fossil fuels, the cost of fueling internal combustion engines is decreased, harmful environmental pollutants are decreased, and/or the future availability of fuels is increased.
Ammonia is one such fuel capable of at least partially replacing crude oil based fuels. Ammonia (NH3) is widely used in household cleaning supplies and agricultural fertilizer. Unlike either gaseous hydrogen or natural gas, ammonia need not be stored under extreme pressures or at cryogenic temperatures to maintain the ammonia at an energy density which is appropriate for use in propulsion applications such as automobiles and boats. Ammonia may be stored indefinitely as an anhydrous liquid at pressures nearly the same as those of propane, approximately 10 bars at 300 Kelvin. Ammonia has reasonable energy/volume and energy/mass densities, which, although lower as those of gasoline by factors of 2.6 and 2.3, respectively, are still well within reach of practical use in automobiles and other machinery as the principal energy carrier. Accordingly, ammonia may be transported via currently available high pressure pipelines. Ammonia may be made from energy sources, such as nuclear power, which is characterized as having a high concentration of energy production per unit of land surface area. A high concentration of energy production is desirable because the costs, of owning or using land, and maintaining equipment, constitute a significant portion of the total cost of energy production. Nuclear power has a high concentration of energy production which is more than 1000 times greater than the 2-3 kilowatts per hectare gross average rate of liquid fuel production, which is typical for the “biofuels”. Therefore, fuel manufacturing, handling, distribution, and use are more feasible for ammonia than for some of the other fuels.
The use of ammonia as an energy carrier makes possible the indirect use of energy obtained from nuclear fission, in mobile applications where direct use would be impractical. In one example, high temperature nuclear reactor heat is used to drive a thermochemical cycle for generating hydrogen, and ammonia is made by combining the hydrogen with nitrogen, obtained by air separation. Hydrogen can be made from high temperature nuclear reactor heat, using the Sulfur-Iodine thermochemical cycle. Ammonia is then made from the hydrogen, using Haber-Bosch ammonia synthesis. A portion of the heat, released by the exothermic formation of ammonia, may be transferred from the Haber-Bosch ammonia synthesis loop to the endothermic hydrogen iodide decomposition process within the Sulfur-Iodine cycle, thereby rendering the combination, of Sulfur-Iodine thermochemical hydrogen production, and Haber-Bosch ammonia synthesis, more efficient for the production of ammonia, than these processes would be if run separately.
In another example, hydrogen is made by high temperature steam electrolysis, using nuclear power to generate the required combination of heat and electricity. Again, ammonia is made from the hydrogen, using Haber-Bosch ammonia synthesis. A presentation, titled “Nuclear-Power Ammonia Production”, presented at the 2006 Ammonia Fuel Conference by Kubic, discloses such a system. In the disclosed system, at least a portion of the heat, released by the exothermic formation of ammonia, is transferred from a Haber-Bosch ammonia synthesis loop to a superheater for a high temperature steam electrolysis unit, thereby rendering the combination, of high temperature steam electrolysis, and Haber-Bosch ammonia synthesis, more efficient for the production of ammonia, than these processes would be if run separately. The conversion of nuclear reactor heat to ammonia, by this or a similar system, can be as high as 48% on a higher heating value basis, or 40% on a lower heating value basis. For further reading, see also a presentation, titled “Nuclear Ammonia—a Sustainable Nuclear Renaissance's ‘Killer Ap’”, presented at the 2011 Ammonia Fuel Conference and authored by Siemer, Sorensen, Hargraves.
Even when natural gas is the chosen primary energy for making ammonia, the conversion, of natural gas to ammonia, may offer some advantages over simply burning the natural gas directly, despite the conversion losses. Some vehicle applications specify the use of liquefied natural gas (LNG). The conversion of natural gas to LNG also involves large energy losses, and LNG requires Dewar tanks and other means of handling, storing and using a cryogen. The use of LNG may also involve significant boil-off and transfer losses, and methane, the principal constituent of natural gas, is a potent greenhouse gas. Means for preventing these losses may be expensive. Also, natural gas and other hydrocarbons can produce carbon monoxide when burned. Therefore, ammonia has value as an engine fuel even when ammonia is made from natural gas. Ammonia is especially valuable for use as a fuel for specialty engines and other appliances which can be safely operated while indoors. A hydrocarbon fueled engine can be run lean or use an exhaust catalyst to avoid carbon monoxide emissions. However, if the air/fuel ratio falls out of adjustment, or if the catalyst becomes nonfunctional, then the hydrocarbon fueled engine may emit dangerously large quantities of carbon monoxide. A diesel engine may emit particulates and malodorous emissions which are unpleasant or hazardous to breathe. Fuel stored as ammonia can be converted to hydrogen, using an Ammonia Flame Cracker. Hydrogen fueled engines, for which the fuel is stored as liquid ammonia, may now be used in applications for which battery/electric was previously the only option, such as forklifts which may be operated indoors. Even barbecuing indoors is feasible with the use of an Ammonia Flame Cracker and appropriate ventilation.
Ammonia can be used as a means of storing and transporting hydrogen for use in other hydrogen-consuming applications disclosed herein or known to the art. In some instances, the costs of transporting hydrogen, and other usability concerns such as tank size and pressure, are sufficient to warrant the purchase of ammonia and the subsequent conversion of ammonia to hydrogen, using an Ammonia Flame Cracker, rather than the purchase of hydrogen as hydrogen.
Like electricity, ammonia is a value-added energy carrier which must be made, with attendant conversion losses, from primary energy. Also like electricity, ammonia can be clean at the point of use, and it can also be made cleanly at the point of manufacture by appropriate choice of primary energy. In some applications, ammonia will be preferred over batteries as the principal means of energy storage, for example, in automobiles and in fishing vessels. Battery/electric systems can be prohibitively expensive and batteries can weigh as much as, or much more, than the rest of the vehicle for an operating range which is currently typical for hydrocarbon fueled vehicles, for example automobiles which are expected to have a range of about 500 kilometers between refueling or recharging. The rechargeable lithium ion battery for a representative electric automobile has a specific capacity of about 53 watt-hours per kilogram for 65% capacity, or 81 watt-hours per kilogram for 100% capacity. If the 197 kilogram battery pack in the representative electric automobile were resealed from a driving range of about 56 kilometers, to a range of 500 kilometers, then the battery pack would have a mass of 1750 kilograms, which is greater than the mass of the rest of the car. Fishing vessels in the 10-20 ton range may carry 4000 liters or more of diesel fuel. For these boats, a rechargeable lithium ion battery would have a mass which is more than 10 times greater than the mass of the rest of the boat, for the same range, according to battery specifications for the representative automobile.
The plant-to-wheels efficiency of an energy chain consisting of, for example, a nuclear reactor equipped with means of making electricity from reactor heat, an electric transmission grid, a battery charger, a battery with attendant charge/discharge losses, and an electric motor with controller (battery/electric) may be either only marginally better or perhaps worse, than the plant-to-wheels efficiency of an energy chain consisting of a nuclear reactor equipped with means of making ammonia from reactor heat, an ammonia distribution network, and an ammonia-fueled internal combustion engine (ammonia/IC engine), the engine possibly incorporating one or more embodiments of the Ammonia Flame Cracker. Even for some cases in which the plant-to-wheels efficiency of battery/electric is substantially greater than the plant-to-wheels efficiency of ammonia/IC engine, ammonia/IC engine may still be preferred due to higher energy storage density, longer range, and lower total cost as compared to battery/electric.
The “yellow coal” limit is the lower bound on the concentration of a fissionable element in rock deemed feasible for mining, such that the mass of rock at the yellow coal limit which must be handled is equal to the mass of coal which must be handled for the same gross energy yield. The “yellow coal” term has been applied to uranium (yellow cake, hence yellow coal), and for the enriched uranium/once through fuel cycle the yellow coal limit is about 70 parts per million (ppm) by weight of natural uranium in rock. Calculations done for thorium fueled breeder reactors indicate a yellow coal limit of about 0.4 ppm by weight of thorium, which is much lower than the estimated 6-12 ppm average concentration of thorium in the earth's crust. Hence the potentially recoverable reserves of primary energy, including thorium, which can be used to cleanly make both ammonia and electricity, are much, much greater than reserves of coal, oil and natural gas combined.
The inventors' U.S. application Ser. No. 12/947,137, filed Nov. 16, 2010, now document number 20110114069, the text and drawings of which are hereby incorporated by reference in its entirety, describe the use of ammonia with oxygen as a combustion promoter to fuel an engine. The inventors' U.S. Pat. No. 7,574,993, the text and drawings of which are hereby incorporated by reference in its entirety, describes the use of ammonia with another fuel for fueling an engine, wherein the other fuel is a combustion promoter. In some cases the combustion promoter was another fuel stored separately from the ammonia. In other cases the combustion promoter was hydrogen. Desirably, the combustion promoter may be derived by using an ammonia cracker to decompose at least a portion of the ammonia into hydrogen and nitrogen, thus enabling engine operation with pure liquid anhydrous ammonia as the only stored fuel.
Ammonia crackers known to the art have difficulties and limitations because of large size and intricate design required for heat transfer, large quantities of sometimes expensive catalyst required to obtain a substantial ammonia decomposition yield, an uncontrolled and often low ammonia decomposition yield, and lack of rapid start capability. Ammonia crackers designed to use engine exhaust heat to decompose ammonia, such as the ammonia crackers disclosed in U.S. Pat. Nos. 2,140,254, 4,478,177, and 4,750,453, are often large, expensive, and intricate devices which must be placed in the engine exhaust flow. Furthermore, an engine's exhaust gas temperature is generally not high enough to decompose any of the ammonia without using an ammonia cracker catalyst. Such cracker catalysts may be large and expensive when sized for providing enough catalytic sites for catalytically decomposing ammonia at a high rate or high decomposition yield. In some instances, an engine's exhaust gas temperature may not be high enough to give acceptable ammonia cracker performance even with the use of a catalyst.
The equilibrium constant determines the maximum possible extent of ammonia decomposition at a given pressure and temperature. The equilibrium constant favors a lower content of ammonia in the mixture, with increasing temperature. The higher temperatures, required for non-catalyzed ammonia decomposition, give a much lower possible concentration of ammonia in a decomposed mixture, than is possible at temperatures at which ammonia cracker catalysts are typically designed to work.
Ammonia crackers may be designed to use electricity to decompose ammonia at high temperatures, including temperatures at which ammonia will decompose rapidly and at a high decomposition yield without the aid of a catalyst (hereafter referred to as the “ammonia cracking temperature”). Examples include electrically heated ammonia crackers disclosed in U.S. Pat. Nos. 1,915,120, 2,013,652, 2,161,746, 2,264,693, 3,025,145, 3,379,507, and 3,598,538. For this discussion the class of electrically powered ammonia decomposers is broadened beyond ammonia crackers using resistively heated elements, to include ammonia decomposers which use electric arcs, electromagnetic energy such as microwaves, or electrolysis to decompose ammonia into hydrogen and nitrogen. However, the conversion of fuel energy into electricity, by an engine system, involves losses in the engine and losses in the generator. Electricity is thus, joule for joule, more costly to use for decomposing ammonia, than is heat obtained by combusting a portion of the ammonia. Some of the Ammonia Flame Cracker (capitalized hereafter and elsewhere to distinguish the present invention from the prior art) embodiments disclosed herein are intended to obtain energy for decomposing ammonia principally from the combustion of some of the ammonia and not from electricity. Therefore, an engine system, incorporating one of the Ammonia Flame Cracker embodiments disclosed herein, will be somewhat more efficient than an otherwise similar engine system incorporating an electrically powered ammonia decomposer.
Even for non-engine applications, it may be preferred to obtain the heat, required to decompose ammonia, by combustion of a portion of the ammonia, rather than by electrical heating, because, in some instances, electricity may be more expensive than ammonia, and also because electrical heating may require an electrical hookup of very substantial capacity at the ammonia cracker, whereas ammonia combustion does not. Furthermore, some applications may be remote. Other applications may be air-born, for example, carried on board balloons, and for these applications the use of very substantial quantities electrical energy may be forbidden.
In some applications, an Ammonia Flame Cracker may be incorporated into an auxiliary system representing only a small fraction of the total energy consumption. Examples include using an Ammonia Flame Cracker to supply hydrogen to an exhaust emissions control device or system, such as one incorporating a hydrogen-SCR (selective catalytic reduction) unit. In these applications, it may be feasible to use electric heating as at least a partial heat input for bringing ammonia up to the ammonia cracking temperature, or for maintaining an Ammonia Flame Cracker at a suitable operating temperature during periods of intermittent cessation or reduction of flow. However, even if electricity is used as at least a partial heat input for bringing ammonia up to the ammonia cracking temperature, it is still both desirable, and an object of the present invention, to derive at least a portion of energy, required to decompose ammonia, by combustion of some of the ammonia. An internal release of heat, at the ammonia cracking temperature, facilitates the gaseous phase decomposition of ammonia in bulk. Once started, the decomposition of ammonia proceeds without additional external application of heat to the gas, and in some cases, without the aid of a catalyst.
Hot filaments, for example glow plugs and the like, are known to the art and may be used for igniting ammonia within a combustion chamber of a piston engine. However, such a filament may prematurely ignite a homogeneous, premixed fuel/air charge, and a large pumping loss would occur if a piston engine were to include a provision for preventing contact between a premixed fuel/air charge and the filament during compression, and another provision for passing the entire charge through the filament region, within a short crank angle duration, when the piston is near top center. The implementation of these provisions within a combustion chamber of a piston engine or other engine with discrete firing cycles is also difficult, burdensome and expensive. Glow plugs are thus unsuitable for use in premixed charge engines with discrete firing cycles. Glow plugs which are used for igniting ammonia are not intended to substantially fully decompose an entire ammonia stream into a hydrogen-containing product mixture, which is destined for combustion or other use elsewhere. Glow plugs are also not generally controlled to operate at a particular local ammonia/air equivalence ratio, and the filament in a glow plug may have a short service life because the adiabatic flame temperature is far in excess of the melting temperature of most common metals when the ammonia/air equivalence ratio is near stoichiometric, as is the case for combusting ammonia and other fuels with air in an engine. “Normal air” (or simply “air”) is defined herein as the natural atmospheric mix of mostly nitrogen and oxygen, which is neither enriched nor depleted in oxygen content.
Ammonia burners, disclosed in U.S. Pat. No. 5,904,910, can combust ammonia with pure oxygen or oxygen-enriched air, or combust ammonia with other fuels and air, or combust ammonia with hydrogen obtained by earlier rich ammonia combustion with oxygen or oxygen-enriched air or with other fuels and air. However, the burners disclosed in U.S. Pat. No. 5,904,910 would not be operable to combust a very rich, homogeneous mixture containing ammonia and oxygen at an equivalence ratio which is richer than the upper flammability limit for ammonia in mixture, without first raising the mixture temperature to the ammonia cracking temperature, and according to specifications, these burners achieve peak temperatures by first combusting the mixture. Some embodiments of the disclosed Ammonia Flame Cracker are operable to decompose ammonia in a single step, using ammonia and normal air as the only inputs, wherein the ammonia/air equivalence ratio is generally much richer than the upper flammability limit for ammonia in air.
Some of the ammonia burners, disclosed in U.S. Pat. No. 5,904,910, can combust ammonia with pure oxygen or oxygen-enriched air. However, the schematic appears to show that ammonia is mixed with an oxygen-containing gas before reaching the burner. The benefits of using a mixing burner, or other burner with separate ammonia and oxygen inputs, are not disclosed in U.S. Pat. No. 5,904,910. The upper flammable limit of ammonia fully premixed in pure oxygen is about 79% ammonia by volume, which corresponds to an ammonia/oxygen equivalence ratio of 2.82. Separate metering of ammonia and oxygen into a mixing burner enables sustained, non-catalytic combustion of ammonia with oxygen at equivalence ratios much greater than 3, thus extending the flammable limits far beyond those possible with a homogeneous mixture. This flammability limit extension can be used to great advantage for decomposing ammonia during warmed up operation of Ammonia Flame Crackers, and also for starting. The benefit of counterflow heat exchanging of the inputs and output of the burner, immediately before the gas is used for anything else, is also not disclosed in U.S. Pat. No. 5,904,910. Heat exchange can be used for obtaining a full ammonia decomposition yield at a higher range of equivalence ratios, thus raising the ammonia-to-hydrogen conversion efficiency. The separate metering, of ammonia and oxygen streams into a mixing burner, enables sustained non-catalytic ammonia combustion and decomposition at equivalence ratios greater than 3, and heat exchange, of the inputs and output of the mixing burner, enables a full ammonia decomposition yield at these equivalence ratios.
U.S. Pat. No. 2,013,809 discloses an ammonia cracker, which decomposes ammonia on a catalyst, which is heated by combustion of products of ammonia decomposition, and at an unspecified ammonia decomposition temperature. U.S. Pat. Nos. 2,601,221 and 2,606,875 describe catalytic ammonia combustion and/or decomposition at temperatures of 500° C. or higher, but these do not further disclose the substantially non-catalyzed combustion and decomposition of ammonia, which occurs at temperatures higher than 1100° C. Other ammonia crackers, such as those disclosed in U.S. Pat. Nos. 2,578,193, 3,505,027, 4,069,071, 4,157,270, 4,179,407, 4,219,528, 5,055,282, 5,139,756, 5,976,723, 6,007,699 and 6,800,386 operate at peak temperatures of 1100° C. or lower, at which the non-catalyzed decomposition of ammonia will not occur at an appreciably high rate. Embodiments of the disclosed Ammonia Flame Cracker are operable to decompose ammonia at a high rate, at peak temperatures which are generally higher than 1100° C., and in a manner which is either substantially or fully non-catalytic.
Systems incorporating ammonia burners and cracker catalysts are disclosed in U.S. Pat. Nos. 4,788,004 and 6,936,363. These systems decompose ammonia on a cracker catalyst, and then combust at least a portion of the decomposed ammonia or other fuels in a burner, which may or may not be catalytic, yielding heat which is used for decomposing more ammonia on the cracker catalyst. However, the systems, disclosed in U.S. Pat. Nos. 4,788,004 and 6,936,363, operate at peak temperatures of about 750° C., at which non-catalyzed decomposition of ammonia will not occur appreciably, thus the cracker catalyst must be sized for catalyzing all of the ammonia decomposition reactions. Some embodiments of the Ammonia Flame Cracker incorporate a catalyst, nevertheless these embodiments are intended to decompose ammonia at peak temperatures higher than 1100° C., at which most of the ammonia decomposes non-catalytically and at a high rate, thus the catalyst need be sized only for starting the reactions and not for catalyzing all of the reactions. Other embodiments of the disclosed Ammonia Flame Cracker are operable without the use of any catalyst. Operation of an Ammonia Flame Cracker without a catalyst is advantageous because some catalysts are expensive, and also because some catalysts may not be durable at temperatures at which ammonia rapidly decomposes non-catalytically.
Systems incorporating endothermic reaction loops heated by exothermic reaction loops are disclosed in U.S. Pat. No. 6,096,106. In basic structure, some of these are very similar to some embodiments of the Ammonia Flame Cracker 600, wherein ammonia or products of ammonia decomposition are combusted in a first exothermic loop and ammonia is decomposed in a second endothermic loop, and a heat exchange relationship exists between the first and second loops. However, the intent of the systems disclosed in U.S. Pat. No. 6,096,106 is the reformation of natural gas or other hydrocarbons, and not the decomposition of ammonia.
Work on cracking ammonia was reported by Lee, Park, and Kwon at the 2008 Ammonia Fuel Conference. The 2008 presentation, delivered on Sep. 29, 2008, is titled “Properties of Laminar Premixed Hydrogen-Added Ammonia/Air Flames”. Additional work on cracking ammonia was reported by Kwon, Too, Lee, and Um at the 2011 Ammonia Fuel Conference. The 2011 presentation, delivered on Sep. 19, 2011, is titled “Reforming and Burning of Ammonia in Micro Hydrogen and Power Generation Systems”. The combined combustor/reformers, shown in these two presentations, appear similar to Ammonia Flame Cracker 600, which has separate ammonia combustion and decomposition loops. However, neither of these two presentations specifies a temperature range for non-catalyzed ammonia decomposition. In particular, no specification is given for the use of temperatures higher than 1100° C. for non-catalyzed decomposition of ammonia.
Based on the foregoing, there is a need for a rapid-starting device for decomposing ammonia into a hydrogen-containing product mixture, said device being characterized as compact and capable of decomposing ammonia at a high decomposition yield, at a high overall thermal conversion efficiency, and at a high throughput rate, using ammonia and normal air as the only inputs, and using very little or no catalyst.